This invention relates to computer animation and more particularly to the use and display of an animation spline, namely the mathematically defined curve in distance and time that is used to produce a smooth path between successive points or key frames in an animation sequence. This invention addresses the problem of intuitively assessing and controlling ease in and ease out for key frames in an animation scene (also known as “slow in and out”). This invention is particularly relevant for monotonically increasing curves, such as those used to control the timing of other animation splines.
With the wide-spread availability of computers, film animators increasingly rely upon computers to assist in the animation process. This process includes using computers to facilitate drawing-based animation, for example, by painting images, by generating in-between images (“tweening”), and the like. This process also includes using computers to augment physical animation techniques. For example, physical models could be represented by virtual models in computer memory, and then the virtual models could be manipulated.
As one of the pioneering companies in the computer aided animation (CAA), Pixar of Emeryville, Calif., has developed computing platforms especially designed for CAA, animation software, and rendering software now known as RENDERMAN®. While RENDERMAN® computer animation/rendering software was focused upon rendering, namely the creation of images from geometric models, the animation software developed for in-house use had focused upon allowing animators to specify (“animate”) the geometric models. The geometric models typically represent objects in a scene, characters in a scene, positions of objects and characters, manipulation of objects and characters, lighting, textures, and the like.
In the process of preparing a fully animated scene, animation splines are provided in display form to guide the animator in making dynamic changes between key frames. Animation splines are typically drawn against a constant distance axis relative to a time axis, typically a vertical axis. As such, an ease in/out effect for a monotonically increasing section takes on an S-curve shape through the knot. This S-curve is non-symmetrical about the vertical axis, making it more difficult for the animator to observe differences between the in and out sections, and as such it is also not especially intuitive to an animator. A further disadvantage of this approach is that splines that represent monotonically increasing quantities over an extended time period require a large amount of vertical space to display fully.
What is needed is a mechanism for enhancing the usefulness of animation splines.